1. Field of the Invention
This invention generally relates to an optical module, and in particular, relates to an optical module coupled to an optical fiber.
2. Description of the Related Art
Recently, an optical module, in which an emitting element such as a laser diode or a receiving element such as photodiode is coupled to an optical fiber, is used in such as an optical communication.
A description will be given of an optical module used conventionally in accordance with a first conventional embodiment with reference to FIG. 1A and FIG. 1B. FIG. 1A illustrates a top view of the optical module in accordance with the conventional embodiment (a cap is not shown). FIG. 1B illustrates a cross sectional view taken along a line A-A of FIG. 1A. A package 20 is fixed to a board used for a heat sink 10, with a screw 27 at a flange 26 as an attachment portion. The heat sink 10 is, for example, composed of Cu or Al. The package 20 has a sidewall 22 and a bottom 24. The bottom 24 is formed with the flange 26 integrally. The sidewall 22 and the bottom 24 are, for example, composed of a KOVAR (trade name) which is Fe—Ni—Co alloy. There is provided the flange 26 at each of the four corners of the bottom 24. An opening 23 is formed at a face of the sidewall 22. An optical fiber 12 is inserted into the package 20. That is, the optical fiber 12 passes through the sidewall 22 of the package 20. The optical fiber 12 is fixed at the sidewall 22 with a first fixing portion 14.
One end of the optical fiber 12 is fixed inside of the package 20 with a second fixing portion 16 having a fixing member 40 and an optical fiber support portion 42. A lens 32 supported with a lens holder 34 and a laser diode 30 arranged on a submount 44 are on an optical axis of the optical fiber 12. The optical fiber 12 is coupled optically to the laser diode 30. The fixing member 40 and the submount 44 are fixed on a base 46. The submount 44 is fixed on a thermo module 48, Thermo Electric cooler (TEC), for keeping temperature constant. The thermo module 48 is fixed on the bottom 24. The optical fiber 12 is fixed to the first fixing portion 14 and a second fixing portion 16. In this case, a distance L is defined as a distance between an edge part of the first fixing portion 14 fixing the optical fiber 12 and an edge part of the second fixing portion 16 fixing the optical fiber 12. The lens holder 34 may be fixed on the base 46, although the lens holder 34 is fixed on the submount 44 in FIG. 1A and FIG. 1B.
The optical fiber 12 is subjected to a stress caused by differentials between linear thermal expansion coefficients of the heat sink 10, the package 20 and the optical fiber 12 (hereinafter, referred to a thermal stress), because the optical module is subjected to an environmental temperature range −40° C. to 85° C. Japanese laid-open Patent Publication No. 2001-100066 (hereinafter, referred to Document 1) discloses that reliability may be degraded because the optical fiber 12 is broken or the optical fiber 12 falls from the fixing portion because of the thermal stress. Document 1 discloses an art (a second conventional embodiment) where the optical fiber 12 has a bending in order to restrain the breaking of the optical fiber 12 or the fall of the second fixing portion 16 fixing the optical fiber 12 from the base 46. FIG. 2 illustrates a cross sectional view of an optical module in accordance with the second conventional embodiment. The same components and configurations as those of FIG. 1A have the same reference numerals and a detailed explanation will be omitted. As shown in FIG. 2, the optical fiber 12 has a bending between the first fixing portion 14 and the second fixing portion 16. In this case, a curvature radius of the optical fiber 12 is referred to r.
In the optical module in accordance with the first conventional embodiment, adding to the above reliability degradation caused by the thermal stress, a decline of an optical coupling ration between the optical fiber 12 and the laser diode 30 also occurs, arising from a change of a relative position between the optical fiber 12 and the laser diode 30 because of the thermal stress. In this case, it is not possible to secure the quality of the optical module. On the other hand, there is a following problem when it is tried to secure the reliability and the quality with a method in accordance with the second conventional embodiment. It is necessary that the curvature radius r of the optical fiber is more than 30 mm, because of an optical or mechanical limitation (30 mm means a limited curvature radius). Therefore, it is necessary to lengthen the distance L between the first fixing portion 14 and the second fixing portion 16, in order to keep the curvature radius of the optical fiber less than 30 mm in the temperature range of the optical module −40° C. to 85° C. It is difficult to downsize the optical module when it is tried to secure the reliability and the quality.